key: cord-1029884-8sh91ztl
authors: Xiao, Sulong; Tian, Zhenyu; Wang, Yufei; Si, Longlong; Zhang, Lihe; Zhou, Demin
title: Recent progress in the antiviral activity and mechanism study of pentacyclic triterpenoids and their derivatives
date: 2018-01-19
journal: Med Res Rev
DOI: 10.1002/med.21484
sha: 4c2e32a9f3d5593712165f1f716bc4dbf549ff7f
doc_id: 1029884
cord_uid: 8sh91ztl

Viral infections cause many serious human diseases with high mortality rates. New drug‐resistant strains are continually emerging due to the high viral mutation rate, which makes it necessary to develop new antiviral agents. Compounds of plant origin are particularly interesting. The pentacyclic triterpenoids (PTs) are a diverse class of natural products from plants composed of three terpene units. They exhibit antitumor, anti‐inflammatory, and antiviral activities. Oleanolic, betulinic, and ursolic acids are representative PTs widely present in nature with a broad antiviral spectrum. This review focuses on the recent literatures in the antiviral efficacy of this class of phytochemicals and their derivatives. In addition, their modes of action are also summarized.

coronavirus (SARS-CoV), 6 Middle East respiratory syndrome coronavirus (MERS-CoV), 7 Ebola virus (EBOV), 8 and so on. Given the high mutation rate of viruses and their serious threat to public health, there is a high urgency to develop new antiviral drugs to combat these pathogens. Natural products in the roots, stems, barks, leaves, fruits, and seeds of plants have been used as herbal medicines to treat diseases for more than 3000 years in China. They also have played a significant role in modern drug discovery by serving as prototypes of novel drugs. As mentioned by Newman et al., 9 it has been estimated that over 49% of clinically used antitumor drugs from around the 1940s to the end of 2014 are natural products or their derivatives. Natural product-derived drugs have also been crucial for antiviral drug discovery.

From 1981 to 2014, over 40% of antiviral drugs in clinical use are natural products (∼6%) or designed using natural products as prototypes (∼34%). 9 Among the natural products, triterpenoids and their relatives, also called the steroids, constitute a very large family of compounds with over 20,000 identified molecules. And many new compounds are being discovered in plants, animals, and fungi every year. 10 Most of them exist in nature as free acids, as esters with fatty acids, ferulic acid, etc., or as triterpenoid saponins linked with one or more sugar chains. They are often the main active constituents of many important medicinal plants, such as the ginsenosides in Panax ginseng, glycyrrhizin in Glycyrrhiza uralensis and saikosaponins in Radix Bupleuri. [11] [12] [13] Triterpenoids display a variety of structures with nearly 200 different skeletons known from natural sources or enzymatic reactions, although many are variants of a smaller number of common structural types. 14 Many pentacyclic triterpenoids (PTs) show a wide range of pharmacological activities, and some are marketed as therapeutic agents or dietary supplements. 15 Structurally, PTs have four six-membered rings called A, B, C, D with ring E being five-membered or six-membered. Based on the carbon skeleton, they are divided into six common subgroups: oleanane, lupane, ursane, friedelane, hopane, and gammacerane ( Fig. 1 ).

PTs are a large class of secondary plant metabolites that are constructed by isoprene (2-methylbutadiene) (C 5 H 8 ) units, an abundant natural product in plants. 16 The linear triterpene squalene, the major constituent of shark liver oil, is derived from two farnesyl pyrophosphate units. Under the catalysis of squalene epoxidase, squalene is biosynthetically F I G U R E 2 Chemical structures of lupeol, -amyrin, and -amyrin F I G U R E 3 Chemical structures of PTs 1-7 oxidized to 2,3-oxidosqualene and various cyclic products, yielding the dammarenyl, lupenyl, and baccharenyl cation intermediates that eventually give rise to three major products namely: lupeol, -amyrin, and -amyrin, respectively ( Fig. 2) . 17 Many PTs have a hydroxyl group at C3, a 12-ene and a carboxylic acid at C17 or C20. The structural diversity of PTs provides a unique group of agents with a variety of pharmacological activities, 18 including antiviral, antitumor, immunomodulatory, anti-inflammatory, and hepatoprotective activities. Presently, the newly discovered PTs are a subject of annual review, reflecting the increasing importance of this class of compounds. 18 The available reviews cover the chemistry, 19, 20 antitumor activity, [21] [22] [23] anti-inflammatory activity, 24 bioavailability, 25 and other pharmacological activity. 18 Recently, the antiviral activities of PTs are attracting increasing attention. In Japan, Stronger Neo-Minophagen C TM (SNMC), a preparation containing glycyrrhizin (glycyrrhizic acid, 1) has been used in the treatment of chronic hepatic diseases for over 40 years. Glycyrrhetinic acid (GA, 2) (Fig. 3) , a PT derivative of the -amyrin type, is obtained from the hydrolysis of glycyrrhizin 1, which is isolated from the herb liquorice and shows inhibition on a variety of viruses including hepatitis B virus (HBV) and HIV. 26 In 1989, saponin B1 and B2, two oleanane-type triterpenoids isolated from soybean seeds, have been reported to have activity against HIV at concentrations greater than 2 and 0.5 mg/mL, respectively. 27 Three years later, Chen et al. reported that salaspermic acid (3), another oleanane-type triterpene derivative isolated from Tripterygium wilfordii, can block the replication of HIV in H9 lymphocytes (IC 50 : 10 M). 28 Afterwards, Fujioka et al. found that betulinic acid (BA, 4), isolated from the leaves and barks of Syzigium claviflorum, also showed significant activity against HIV (EC 50 : 1.4 M). 29 Since then, more and more natural PTs have been reported to have antiviral activity. It is well known that lipids, a class of hydrophobic biomolecules, are important for cellular life, especially during virus entry into the host cell. It was reported that the promiscuous antiviral activities of natural PTs might be related to the effect of lipid metabolism. 30, 31 Synthetic chemists become more interested in these scaffolds due to their potential antiviral properties, leading to the production of a variety of PT derivatives through modifications of the C3-OH, C17-COOH, and other functional groups. Some of these derivatives are used as clinical drugs for the treatment of liver-related diseases, while others are at various phases of clinical trials. 20 For example, the BA derivative Bevirimat (BVM, 5), also known as 3-O-(3 ′ ,3 ′dimethylsuccinyl) BA (DSB) or PA-457, was found to exhibit remarkable anti-HIV activity against primary and drugresistant HIV isolates. 32 It represents a unique first in class anti-HIV compound termed maturation inhibitors (MIs). 33 Due to the baseline gag polymorphisms at positions Q369, V370, or T371, which reduced the sensitivity of HIV-1 to Bevirimat (5), it was halted for further development in 2010. 34, 35 Recently, the second class of potent PTs BMS-955176 (6) and GSK-2838232 (7) are under development as oral treatment of HIV infection. [36] [37] [38] [39] However, up to now, there is no review focusing on the activity of PTs against viral infection. 40 The focus of the present review is to summarize the recent advances in the antiviral activities and action models of PTs to aid the antiviral drug development of PTs.

HIV is a lentivirus that could lead to HIV infection and over time acquired immunodeficiency syndrome (AIDS). 41 Because of the lack of CD4-positive T cells, HIV patients are exposed to life-threatening opportunistic infections and cancers without normal immune protection. 42, 43 Fortunately, HIV-1 infection can be effectively managed with a longterm administration of multiple antiviral inhibitors designated as combination antiretroviral therapy (cART). 44 Since Zidovudine (ZDV, also known as azidothymidine, AZT) was launched in 1987 as the first anti-HIV drug that targets HIV reverse transcriptase, so far over 40 formulations have been approved by FDA for HIV therapy. However, none of them is PTs. According to Thomson Reuters, among all chemicals in clinical trials, only four belong to PTs. They are MPC-9055 (the structure has not been reported in literature), Bevirimat (5), BMS-955176 (6), and GSK-2838232 (7) . These four compounds are all MIs bearing the scaffold of BA (4). GSK-2838232 (7), an orally active BA derivative, targets against HIV-1 maturation process and it is under phase IIa clinical trial. However, the clinical testing of MPC-9055, Bevirimat (5) , and BMS-955176 (6) have been discontinued. [36] [37] [38] [39] Many PTs and their derivatives have been reported to have fair anti-HIV activity. Most of them usually show weaker anti-HIV activity than the other clinically available drugs. 45 However, they enrich the database of anti-HIV compounds and provide good lead compounds for further drug development due to their diverse chemical structures. According to the life cycle of HIV, the inhibitors of HIV are divided into entry inhibitor, reverse transcriptase (RTase) inhibitor, protease inhibitor, viral MI, and so on. 46 Table 1 summarizes the representative anti-HIV PTs based on their targets.

Entry of HIV-1 into host cells, the first phase of the viral replication cycle, is an ordered multistep process that involves the attachment of virus to the host cell, the binding of virus to the co-receptor, and the fusion of the cell and viral membranes. HIV-1 uses multiple pathways for entry, 47 such as usurpation of cellular transport pathway, [48] [49] [50] surfing along the cell surface. 48, 50 These processes provide targets for developing novel entry inhibitors to prevent HIV infection in the first step.

at the concentration of 0.16 mM in MT-4 and MOLT-4 cells. 51 However, the underlying mechanism of this kind of compounds remains unclear. RPR103611 (9), a BA derivative with aminooctanoyl amino-3(S)-hydroxy-6-methylheptanoic acid side chain at C17-COOH, has been shown to be a potent inhibitor against envelope-mediated membrane fusion of HIV-1. 53 IC 50 values of compound 9 varied in the range of 40-100 nM in most cell systems, with selective indexes in excess of 100. The transmembrane glycoprotein gp41 was initially reported to be the target of RPR103611 (9), because the I84S mutation of gp41 of CXCR4-dependent HIV-1 LAI was sufficient for drug resistance against RPR103611 (9) . 54 Further study illustrated that both the sequence of gp41 loop and the stability of the gp120-gp41 complex can affect the antiviral efficacy of RPR103611 (9). 55 IC9564 (10), the stereoisomer of RPR103611 (9) , was found to be equally active with an EC 50 value of 0.4 M. 56 Through interacting with gp120 and gp41 and by changing the structure of acceptor gp41, the two BA derivatives 9 and 10 can block the adsorption of HIV to host cells. In addition, compound 9 and its isomer 10 showed the highest activity in the MAGI and fusion assays of H9 lymphocyte.

HIV RTase catalyzes the replication of virus RNA into DNA. Inhibition of RTase enzyme disrupts this essential process for virus replication and infection. Therefore, RTase has long been an obvious target in the battle against HIV.

Mimusopic acid (11), a triterpene analogue isolated from the seeds of Mimusops elengi, has been reported to possess weak HIV-RTase inhibitory activity with IC 50 at micromolar level. 57 Pengsuparp et al. reported that 1-betahydroxyaleuritolic acid 3-p-hydroxybenzoate (12) , isolated from the roots of Maprounea africana, also had anti-HIV-1

RTase activity with an IC 50 value of 3.7 M. 58

The HIV polyprotein that contains several viral proteins joined together must be cleaved into the individual functional proteins. This cleavage is catalyzed by HIV protease, a symmetric homodimer. Inhibition of HIV protease can prevent the enzyme from cutting the viral protein molecules down to their proper sizes, which makes HIV protease a promising drug target.

Ursolic acid (UA, 13) and its hydrogen malonate (14) isolated from the stems of Cynomorium songaricum have activity in inhibiting HIV-1 protease with EC 50 value at micromolar level. The glutaryl hemiesters 15-16 of BA and UA were shown to have anti-HIV-1 protease activity at the concentration of 4 M. 59 Oleanolic acid (OA, 17) from Xanthoceras sorbifolia was found to completely inhibit HIV-1 protease activity at the concentration of 100 g/ L. 60 Chemical modifications of oleanane-type triterpene at C3-OH and/or C17-COOH increase the anti-HIV protease activity. The most potent compounds 18-20 showed anti-HIV protease activity with IC 50 s in the range of 1.7-4.0 M. 61 In addition, escin Ia (21) and Ib (22) , two triterpenes saponins isolated from Aesculus chinensis, also exhibit HIV-1 protease inhibitory activity. 62 The introduction of a 3,3-dimethylsuccinyl group at the hydroxyl of C3 of OA (17) generated the most potent compound 23 3-O-3 ′ ,3 ′ -dimethylsuccinate that has greatly increased the potency without significant cytotoxicity. Compound 23 was shown to have anti-HIV-1 activity at subnanomolar concentrations with a therapeutic index of 22,400. 65 Similarly, the introduction of a 3,3-dimethylsuccinyl group at the same position of BA (4) leads to the discovery of Bevirimat (5). Compound 5 greatly increased the anti-HIV activity of BA with an IC 50 value of less than 0.35 nM and a selective index of more than 20,000. 66 MPC-9055 (the structure has not been reported in literature), an orally bioavailable small molecule for HIV therapy, was developed by Myrexis. It blocks HIV-1 replication by interrupting the processing of the viral capsid protein 25 to p24, which condenses to form the conical core structure surrounding the viral genome. A phase I clinical trial of MPC-9055 in healthy volunteers was completed in 2008. The primary objective of the study was to evaluate the safety, tolerability, and pharmacodynamics of MPC-9055. Results show that the overall safety profile was favorable and the observed Pharmacokinetics (PK)/Pharmacodynamics (PD) profile supported continued research and development. 67 First-generation MIs are ineffective against some naturally occurring variations (polymorphisms) in the Gag protease polyprotein; however, the second-generation inhibitors have been developed to better tolerate gag polymorphisms. 68 

HCV is an enveloped, positive-stranded RNA virus in the genus hepacivirus, and belongs to the flaviviridae family. There are at least six distinct genotypes with multiple subtypes in each genotype class, which is important for the prediction of therapy response. 70, 71 HCV has an error-prone replication process for the RNA genome of about 9.6 kb in full length.

It encodes a single polyprotein of more than 3000 amino acids, which is cleaved by viral and host enzymes into ten mature individual proteins, including three structural proteins (core, E1, E2) and seven regulatory proteins (p7, NS2, NS3, NS4A, NS4B, NS5A, NS5B). 72 According to Thomson Reuters, more than 100 anti-HCV drugs have been launched.

However, none of them is PTs. Only ME 3738 (35), a PT, had been in Phase I clinical trials. 73 Table 2 summarizes the representative anti-HCV PTs based on their targets.

It was proposed that HCV glycoproteins could be essential drug targets for the development of HCV entry inhibitors. [74] [75] [76] [77] The core structure of E2 protein was solved in 2013. As a result, many agents are investigated as anti-E2 inhibitors to interrupt the interaction between E2 and CD81, thus inhibiting HCV entry. 78, 79 Small molecules are designed and synthesized to mimic the interacting space and hydrophobic feature of the helix D region of CD81, TA B L E 2 Summary of anti-HCV PTs based on their targets

Entry inhibitors OA (17) EA (26) and its derivatives (27) (28) (29) (30) (31) (32) Protease inhibitors OA (17) and its derivatives (33) (34) Immunomodulator ME 3738 (35) Other inhibitor Glycyrrhizin (1) with an IC 50 value of about10 nM. 85 A series of bivalent oleanane-type triterpenes were further designed and synthesized by optimizing the hydrophobicity, rigidity, and length of the linker. It was found that compound 29 exhibited dramatically enhanced activity with an IC 50 value in the subnanomolar ranges. 86 It is well known that the low solubility in biological matrixes and the high hydrophobicity of PTs hinder their applications, particularly in the development of therapeutic agents. Given the limited solubility of PTs, a series of triterpenecyclodextrin (CD) conjugates were synthesized to improve the solubility and maintain the activity of triterpenes. 87, 88 Based on HCVpp/VSVGpp entry assays, the IC 50 To elucidate the anti-HCV pharmacophore of EA (26), a microbial transformation strategy and a ring expansion/opening strategy have been used to explore the SAR. 89, 90 Results showed that most of the metabolites generated by microbial transformation did not exhibit improved anti-HCV activity except compounds 31 and 32, which displayed similar or even a little higher potency than 26 in HCV entry assay. Meanwhile, the ring expansion and ring opening derivatives of EA (26) , such as lactones, 3,28-dioic acids, or pentols, showed no improved anti-HCV activity. These studies showed that rings A and C of EA are highly conserved and chemical modification of the two rings abolishes the potency of 26, suggesting that the steric hinder effects of the rigid skeleton play an important role in determination of the anti-HCV entry activity.

In summary, chemical exploration of these triterpene compounds revealed that ring D, right side of ring E, and C17-COOH are tolerant to modifications, while ring A, B, C and the left side of ring E are highly conserved for the anti-HCV entry activity. Introducing a hydroxyl group in ring D at C-16 can enhance the potency of triterpene and remove the hemolytic effect of EA (26) (Fig. 6 ).

OA (17) was also found to be an HCV protease inhibitor. 91 Based on this lead compound, 29 OA derivatives were synthesized and tested for their inhibitory activity on hepatitis C viral protease. The derivatives of dicarboxylic acid hemiesters showed potent activity against HCV protease. Of the dicarboxylic acid hemiesters, two compounds 34 and 35 with relatively short carbon chains showed much lower cytotoxicity than OA but maintained the inhibitory activity on HCV protease.

22 -methoxyolean-12-ene-3 ,24(4 )-diol (ME 3738, 35), a derivative of soyasapogenol B, inhibits HCV replication by inducing oligoadenylate synthetase 1 gene expression and enhancing the effect of interferon (IFN)-to increase IFN-stimulated gene expression. In vitro assay, 35 reduces HCV core antigen (HCVcAg) and HCV-RNA levels in mouse livers. 92 Moreover, ME 3738 was also found to be an inducer of interleukin 6 (IL-6). It can stimulate the production of IL-6 and protect mouse from acute liver injury/failure induced by concanavalin A (Con A) and prevent the development of alcoholic fatty liver. 93, 94 Phase I trial of 35 was carried out in 2001 and it showed that ME 3738 strongly reduced positive and negative RNA strand levels, as measured by real-time Polymerase Chain Reaction (PCR).

Glycyrrhizin (1) 

Glycyrrhizin (1) has been isolated from the roots of licorice, and showed a broad antiviral spectrum. 109 (Table 3 ). More importantly, the lead compound 43 shows no tendency to induce resistance of influenza viruses, suggesting that it may have the potential to overcome antiviral drug resistance.

The exploration of the mechanism of action indicated that 43 could block the attachment of influenza virus particles to the membrane of host cells by binding to sialic acid receptor-binding pocket on the influenza viral HA protein (Fig. 8 ).

Tri-or multivalent ligands show much higher binding affinity to the homotrimeric influenza HA receptor. Recently, our group designed and synthesized a series of multivalent PT--( -, -) CD conjugates (45) (46) (47) (48) to enhance the affinity between PTs and HA and thus improve their potency against influenza virus. 122, 123 The multivalent OA--CD conjugate 46 was found to show the most potent antiviral activity against influenza A virus (IC 50 : 1.6 M), more potent than the monovalent OA--CD conjugate 49 by 60-fold (IC 50 > 100 M). Further study on its anti-influenza action mechanism showed that the multivalent PT-CD conjugates could also bind to influenza viral HA protein, and thus inhibit the attachment of influenza virus particle to host cells (Fig. 9) .

Recently 

Among PTs, glycyrrhizin (1) and its derivatives were first reported to have the anti-SARS-CoV activity by Cinatl et al. 110 Inhibitor 1 presented an EC 50 were effective against SARS-CoV with EC 50 at micromolar level. But these derivatives also demonstrated severe cytotoxicity, resulting in decreased selectivity index ranging from 2 to 5. These results showed that the chemical embellishment by the introduction of CONH bonds into 1 was able to enhance its antivirus effectiveness. Hover also found that 2 (GA), the partial hydrolyzate of 1, exhibited high activity as well as toxicity. Considering the mechanism of their anti-SARS-CoV activity, Hover speculated that the entry process of SARS-CoV into cells was blocked by the attachment of N-acetylglycosamine with the carbohydrates of the S-proteins, 111 which was demonstrated to be necessary for viral entry into host cells. (Table 4) . 136 Comparing with 4 whose EC 50 

Ryu and co-workers isolated four quinone-methide triterpenes (67) (68) (69) (70) from Triterygium regelii. Compound 67 was smoothly converted to its derivatives 71 by hydrogenation under palladium-carbon catalyst. 137 The five compounds were tested for SARS-CoV 3CL protease inhibitory activity ( Table 5) Glycyrrhizin (1) has been used for treating HBV for many years. In vitro and in vivo experiments have been conducted to test its mechanism of action. 143, 144 The study showed that treatment with 1 can inhibit the secretion of hep- The Tibetan herb Potentilla anserine L. has been widely used in China to treat hepatitis B. Compound 73 (Fig. 11) (Fig. 12) , from this herb and evaluated their anti-HSV activity in vitro and in vivo. 156 The effective concentrations for 50% plaque reduction of wild-type HSV type 1 (HSV-1) by 66 and 74 were 5.7 M and 8.6 M, respectively. The therapeutic index of 74 (10.3-16. 3) was larger than that of 66 (6. 2). Susceptibility of Acyclovir phosphonoacetic acid-resistant HSV-1, thymidine kinase-deficient HSV-1, and wildtype HSV type 2 to 74 was similar to that of the wild-type HSV-1, indicating that 74 has a broad antiviral spectrum against HSV.

Ikeda et al. studied the SAR of 15 oleanane-type triterpenoids including glycyrrhizin (1) and its sapogenol against HSV-1. 157 The results showed that glycyrrhetic acid (2) and the sapogenol of glycyrrhizin (1) had much more potent anti-HSV-1 activity than glycyrrhizin (1). In addition, it was found that soyasapogenol A (75) showed less anti-HSV activity than soyasapogenol B (76) by 20-fold, suggesting that hydroxylation at C-21 might weaken antiviral activity against HSV-1. It was also found that kudzusapogenol A (77), abrisapogenol B (78) , and abrisapogenol C (79) had no anti-HSV-1 activity, suggesting that the C-29 hydroxyl group could abolish the activity against HSV-1. However, kudzusapogenol B (80) and the methyl esters of 2 (81 and 82) exhibited stronger anti-HSV activity, suggesting that a methyl ester group at C-30 could improve the anti-HSV-1 activity.

Furthermore, Gong et al. determined the synergistic anti-HSV-1 effect of betulin (83) and Acyclovir in a drug combination study. 158 The potent and moderate synergistic anti-HSV-1 effects were found for Acyclovir and betulin (83) when their concentrations were higher than 4.4 M and 0.9 M, respectively. The synergistic anti-HSV-2 effects were also found when the concentrations of Acyclovir and betulin were 2.0 M and 19.0 M, respectively. The synergistic antiviral effect suggested that the action mechanism of betulin is different from that of Acyclovir. Glycyrrhizin (1), as one of the main component isolated from Glycyrrhiza spp., has many kinds of antiviral activity.

Wang's study first recognized the medicinal effectiveness of 1 against EV71. 160 Compound 1 can inhibit EV71 replication in a dose-dependent manner and the concentrations required for 1 to inhibit EV71 infection were in the millimolar ranges. 1 reduced infectious EV71 production by 34% and 60% at 3 mM and 5 mM, respectively. In addition to the iden- 161 The SI value of greater than 200, strong activity and low cytotoxicity indicated its potential use for treating infection of EV71 (Table 6) . Furthermore, the action of 12 against EV71 was from −2 to 24 hr, indicating that its inhibitory effect occurred in the infection and replication process. Therefore, it may have the potential preventive effect as well as treatment effect.

Song et al. reported that hederasaponin B (84) (Fig. 13) from Hedera helix has the anti-EV71 activity. 162 Compound 84 could significantly decrease the formation of visible CPE in Vero cells, demonstrating its effective anti-EV71 C3 and C4a activity without obvious cytotoxicity (Table 7 ). In addition, western blot assay showed that 84 was able to reduce the expression of viral VP2 protein, thus demonstrating the inhibitory effect of the synthesis of viral capsid protein. By comparison, Ribavirin showed no anti-EV71 C3 and C4a activity. These results indicated that 84 is a promising drug candidate against various subgenotypes of EV71 because of its broad spectrum of antiviral activity.

Zhao et al. evaluated six newly synthesized PTs for their anti-EV71 activity. They analyzed their inhibitory effects on the expression of VP1 protein through western blot analysis and reverse transcription (RT) PCR. 163 Compounds 85, 86, and 87 demonstrated more potent anti-EV71 activity than their parent compounds while no significant cytotoxicity was found.

Epstein-Barr virus (EBV) is one of the major pathogens of infectious mononucleosis in humans. It is generally linked with two malignancies, endemic Burkitt's lymphoma and nasopharyngeal carcinoma (NPC). The number of EBVassociated diseases is increasing, requiring the development of effective vaccines for protection as well as novel antiviral agents for treatment.

Lin et al. reported that glycyrrhizin (1) exhibited activity against EBV replication in superinfected Raji cells in a dosedependent manner (IC 50 = 0.04 mM, CC 50 = 4.8 mM, SI = 120). 164 Time of addition studies showed that the anti-EB effect of 1 was closely associated with an event post virus cell entry rather than the direct inactivation of the virus and the blocking of virus adsorption. The antiviral mechanism appeared to be at the early step of EBV replicative cycle.

Different from the mechanism of nucleoside analogs targeting viral DNA polymerase, 1 and its derivatives represent a new type of anti-EBV agents.

Chang and co-workers first reported moronic acid (76) , which was isolated from the galls of Brazilian propolis and Rhus chinensis, as an agent against EB virus. 165 Compared with other triterpenes, 76 has a novel anti-EB mechanism.

During the immediate-early stage of the lytic cycle of EBV, the transcription of viral lytic genes can be activated by Rta 

PTs are a class of plant metabolites with high structural diversity. They provide important sources of lead compounds in drug research and development. Some PTs and their derivatives have been used to prevent and treat chronic hepatic diseases in many countries, such as China and Japan. In addition, they generally show no or weak toxicity. However, the value of PTs in preventing and treating viral diseases has only partially been exploited. Up to now, only Bevirimat (5) The investigation of the molecular basis of their antiviral activity is also progressing rapidly. However, their precise targets and action mechanism are usually unclear, which limits their further development. In addition, the poor aqueous solubility is another major drawback for various applications, in particular, the development of drugs. Those intensive explorations will lead to the design of new PTs as potent antiviral inhibitors with better antiviral activity profiles and lower IC 50 values. We also like to express our regrets not all relevant researches can be cited in this review due to limited space.

http://orcid.org/0000-0002-6852-9504

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The emergence and diversification of panzootic H5N1 influenza viruses

Emergence and development of H7N9 influenza viruses in China

Identification of a novel coronavirus in patients with severe acute respiratory syndrome

Isolation of a novel coronavirus from a man with pneumonia in Saudi Arabia

The evolution of Ebola virus: insights from the 2013-2016 epidemic

Natural products as sources of new drugs from 1981 to 2014

Triterpenoids and rexinoids as multifunctional agents for the prevention and treatment of cancer

Radix bupleuri: a review of traditional uses, botany, phytochemistry, pharmacology, and toxicology

A review of panax ginseng as an herbal medicine

Pharmacological effects of glycyrrhiza spp. And its bioactive constituents: update and review

On the origins of triterpenoid skeletal diversity

Beneficial health effects of lupeol triterpene: a review of preclinical studies

The isoprene rule and the biogenesis of terpenic compounds

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Synthesis, biology and clinical significance of pentacyclic triterpenes: a multi-target approach to prevention and treatment of metabolic and vascular diseases

Chemistry, biological activity, and chemotherapeutic potential of betulinic acid for the prevention and treatment of cancer and HIV infection

Betulinic acid and its derivatives as potential antitumor agents

Ursane-type pentacyclic triterpenoids as useful platforms to discover anticancer drugs

Ursolic acid and oleanolic acid: pentacyclic terpenoids with promising anti-inflammatory activities

Pentacyclic triterpene bioavailability: an overview of in vitro and in vivo studies

Glycyrrhizic acid inhibits virus growth and inactivates virus-particles

Inhibitory effect of glycosides like saponin from soybean on the infectivity of HIV in vitro

Anti-AIDS agents .6. Salaspermic acid, an anti-HIV principle from tripterygiumwilfordii, and the structure-activity correlation with its related-compounds

Anti-AIDS agents .11. Betulinic acid and platanic acid as anti-HIV principles from syzigium-claviflorum, and the anti-HIV activity of structurally related triterpenoids

Lipid interactions during virus entry and infection

Interplay between hepatitis C virus and lipid metabolism during virus entry and assembly

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Pa-457: a potent HIV inhibitor that disrupts core condensation by targeting a late step in gag processing

Phase I and II study of the safety, virologic effect, and pharmacokinetics/pharmacodynamics of single-dose 3-O-(3 ′ ,3 ′ -dimethylsuccinyl)betulinic acid (bevirimat) against human immunodeficiency virus infection

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Choosing initial antiretroviral therapy: current recommendations for initial therapy and newer or investigational agents

Identification and characterization of BMS-955176, a second-generation HIV-1 maturation inhibitor with improved potency, antiviral spectrum, and gag polymorphic coverage

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Human immunodeficiency virus induces phosphorylation of its cell surface receptor

Triterpene derivatives that block entry of human immunodeficiency virus type 1 into cells

Resistance to a drug blocking human immunodeficiency virus type 1 entry (RPR103611) is conferred by mutations in gp41

Sensitivity to a nonpeptidic compound (RPR103611) blocking human immunodeficiency virus type 1 Env-mediated fusion depends on sequence and accessibility of the gp41 loop region

Anti-AIDS agents 49. Synthesis, anti-HIV, and anti-fusion activities of IC9564 analogues based on betulinic acid

Triterpenoid saponins of Mimusops elengi

Mechanistic evaluation of new plant-derived compounds that inhibit HIV-1 reverse transcriptase

Inhibitory effects of constituents from Cynomorium songaricum and related triterpene derivatives on HIV-1 protease

Inhibitory effects on HIV-1 protease of constituents from the wood of Xanthoceras sorbifolia

Chemical modification of oleanene type triterpenes and their inhibitory activity against HIV-1 protease dimerization

Anti-HIV-1 protease triterpenoid saponins from the seeds of Aesculus chinensis

Potent activity of the HIV-1 maturation inhibitor bevirimat in SCID-hu Thy/Liv mice

Anti-viral characterization in vitro of a novel maturation inhibitor

Anti-HIV activity of oleanolic acid, pomolic acid, and structurally related triterpenoids

Anti-AIDS agents-XXVII. Synthesis and anti-HIV activity of betulinic acid and dihydrobetulinic acid derivatives

Phase 1 single ascending oral dose study of the safety, tolerability, and pharmacokinetics of a novel HIV-1 maturation inhibitor in HIV negative, healthy volunteers, poster 570

Pharmacological intervention of HIV-1 maturation

Bifunctional anti-human immunodeficiency virus type 1 small molecules with two novel mechanisms of action

Interferon alfa-2b alone or in combination with ribavirin as initial treatment for chronic hepatitis C

Randomised trial of interferon alpha2b plus ribavirin for 48 weeks or for 24 weeks versus interferon alpha2b plus placebo for 48 weeks for treatment of chronic infection with hepatitis C virus. International Hepatitis Interventional Therapy Group (IHIT)

Hepatitis C virus infection

Clinical efficacy of combination therapy with ME3738 and pegylated interferonalpha-2a in patients with hepatitis C virus genotype 1

Characterization of the envelope glycoproteins associated with infectious hepatitis C virus

Alanine scanning mutagenesis of hepatitis C virus E2 cysteine residues: insights into E2 biogenesis and antigenicity

Ultrastructural and biophysical characterization of hepatitis C virus particles produced in cell culture

Characterization of fusion determinants points to the involvement of three discrete regions of both E1 and E2 glycoproteins in the membrane fusion process of hepatitis C virus

The tetraspanin web modulates immune-signalling complexes

Small molecule inhibition of hepatitis C virus E2 binding to CD81

A novel small molecule inhibitor of hepatitis C virus entry

Discovery of highly potent small molecule hepatitis C virus entry inhibitors

Screening of small-molecule compounds as inhibitors of HCV entry

Ursolic acid-A pentacyclic triterpenoid with a wide spectrum of pharmacological activities

Development of oleanane-type triterpene as a new class of HCV entry inhibitors

Development of bivalent oleanane-type triterpenes as potent HCV entry inhibitors

Synthesis and anti-HCV entry activity studies of beta-cyclodextrin-pentacyclic triterpene conjugates

Synthesis and biological evaluation of novel pentacyclic triterpene alpha-cyclodextrin conjugates as HCV entry inhibitors

Elucidation of the pharmacophore of echinocystic acid, a new lead for blocking HCV entry

Synthesis and biological evaluation of ring A and/or C expansion and opening echinocystic acid derivatives for anti-HCV entry inhibitors

HCV protease inhibitory, cytotoxic and apoptosis-inducing effects of oleanolic acid derivatives

ME3738 enhances the effect of interferon and inhibits hepatitis C virus replication both in vitro and in vivo

Effect of the inducer of interleukin-6 (ME3738) on rat liver treated with ethanol

ME3738 protects from concanavalin A-induced liver failure via an IL-6-dependent mechanism

Glycyrrhizin as antiviral agent against hepatitis C virus

Antiviral agents active against influenza A viruses

Are we ready for pandemic influenza?

Influenza pandemics of the 20th century

Human infection by avian influenza a H5N1

Influenza A/H5N1 virus infection in humans in Cambodia

Of chickens and men: avian influenza in humans

Anti-influenza drug discovery: structure-activity relationship and mechanistic insight into novel angelicin derivatives

Health and economic benefits of early vaccination and nonpharmaceutical interventions for a human influenza A (H7N9) pandemic: a modeling study

Isolation of a novel swine influenza virus from Oklahoma in 2011 which is distantly related to human influenza C viruses

Clinical characteristics are similar across type A and B influenza virus infections

Quantifying influenza virus diversity and transmission in humans

Incidence of adamantane resistance among influenza A (H3N2) viruses isolated worldwide from 1994 to 2005: a cause for concern

Chemical modification of glycyrrhizic acid as a route to new bioactive compounds for medicine

Glycyrrhizin, an active component of liquorice roots, and replication of SARS-associated coronavirus

Antiviral activity of glycyrrhizic acid derivatives against SARS-coronavirus

Glycyrrhizic acid inhibits influenza virus growth in embryonated eggs. Microbiologica

Glycyrrhizin inhibits influenza A virus uptake into the cell

Glycyrrhizic acid derivatives as influenza A/H1N1 virus inhibitors

Uralsaponins M-Y, antiviral triterpenoid saponins from the roots of Glycyrrhiza uralensis

Structure-activity relationships of 3-O-beta-chacotriosyl ursolic acid derivatives as novel H5N1 entry inhibitors

Structure-activity relationships of 3-O-beta-chacotriosyl oleanane-type triterpenoids as potential H5N1 entry inhibitors

Structure-activity relationships of 3-O-beta-chacotriosyl oleanic acid derivatives as entry inhibitors for highly pathogenic H5N1 influenza virus

3-O--chacotriosyl benzyl ursolate inhibits entry of H5N1 influenza virus into target cells

Discovery of the first series of small molecule H5N1 entry inhibitors

Discovery of pentacyclic triterpenoids as potential entry inhibitors of influenza viruses

Pentacyclic triterpenes grafted on cd cores to interfere with influenza virus entry: a dramatic multivalent effect

Inhibition of influenza virus infection by multivalent pentacyclic triterpene-functionalized per-O-methylated cyclodextrin conjugates

Methylated cycloamyloses (cyclodextrins) and their inclusion properties

Cyclodextrin and Their Inclusion Complex. Budapest: Akadémiai Kiado

Conjugation of cyclodextrin with fullerene as a new class of HCV entry inhibitors

Design, synthesis and biological evaluation of novel L-ascorbic acid-conjugated pentacyclic triterpene derivatives as potential influenza virus entry inhibitors

Synthesis and in vitro anti-influenza virus evaluation of novel sialic acid (C-5 and C-9)-pentacyclic triterpene derivatives

Synthesis of novel pentacyclic triterpene-neu5ac2en derivatives and investigation of their in vitro anti-influenza entry activity

Design, synthesis and biological activity evaluation of novel conjugated sialic acid and pentacyclic triterpene derivatives as anti-influenza entry inhibitors

Anti-influenza activity of betulinic acid from Zizyphus jujuba on influenza A/PR/8 virus

Antiviral triterpenoids from the medicinal plant Schefflera heptaphylla

An anti-influenza component of the bark of Alnus japonica

Synthesis of triterpenoid acylates-an effective reproduction inhibitors of influenza A (H1N1) and papilloma viruses

Small molecules targeting severe acute respiratory syndrome human coronavirus

Specific plant terpenoids and lignoids possess potent antiviral activities against severe acute respiratory syndrome coronavirus

SARS-Cov 3CLpro inhibitory effects of quinone-methide triterpenes from Tripterygium regelii

Hepatitis B virus mutants and fulminant hepatitis B: fitness plus phenotype

A dominant hepatitis B virus population defective in virus secretion because of several S-gene mutations from a patient with fulminant hepatitis

Novel viral and host targets to cure hepatitis B

Treatment of hepatitis B

Betulinic acid-mediated inhibitory effect on hepatitis B virus by suppression of manganese superoxide dismutase expression

Effects of glycyrrhizin on hepatitis B surface antigen: a biochemical and morphological study

Therapeutic basis of glycyrrhizin on chronic hepatitis B

Effect of artemisinin/artesunate as inhibitors of hepatitis B virus production in an "in vitro" replicative system

Anti-hepatitis B virus activities of triterpenoid saponin compound from Potentilla anserine L

Synthesis and cytotoxic activity of 17-carboxylic acid modified 23-hydroxy betulinic acid ester derivatives

Triterpenoids from Pulsatilla chinensis

The nine ages of herpes simplex virus

Herpes simplex virus infections

Antiviral drugs in current clinical use

Resistant herpes simplex virus type 1 infection: an emerging concern after allogeneic stem cell transplantation

Resistance to antiviral drugs in herpes simplex virus infections among allogeneic stem cell transplant recipients: risk factors and prognostic significance

Antiviral traditional medicines against herpes simplex virus (HSV-1), poliovirus, and measles virus in vitro and their therapeutic efficacies for HSV-1 infection in mice

Suppression of recurrent genital herpes simplex virus type 2 infection by Rhus javanica in guinea pigs

Anti-herpes simplex virus activity of moronic acid purified from Rhus javanica in vitro and in vivo

Anti-herpes virus type 1 activity of oleanane-type triterpenoids

The synergistic effects of betulin with acyclovir against herpes simplex viruses

Pentacyclic triterpenes in birch bark extract inhibit early step of herpes simplex virus type 1 replication

Glycyrrhizic acid as the antiviral component of Glycyrrhiza uralensis fisch. Against coxsackievirus A16 and enterovirus 71 of hand foot and mouth disease

Antiviral activities of extracts and selected pure constituents of Ocimum basilicum

Antiviral activity of hederasaponin B from Hedera helix against enterovirus 71 subgenotypes C3 and C4a

Inhibition of human enterovirus 71 replication by pentacyclic triterpenes and their novel synthetic derivatives

Mechanism of action of glycyrrhizic acid in inhibition of Epstein-Barr virus replication in vitro

Inhibition of the Epstein-Barr virus lytic cycle by moronic acid

Recent progress in the antiviral activity and mechanism study of pentacyclic triterpenoids and their derivatives